Division of Radiation and Cancer Biology, Department of Radiation Oncology, University of Michigan, 4424B MS-1, 1301 Catherine Street, Ann Arbor, MI 48109, USA.
Division of Radiation and Cancer Biology, Department of Radiation Oncology, University of Michigan, 4424B MS-1, 1301 Catherine Street, Ann Arbor, MI 48109, USA.
Neoplasia. 2015 Jan;17(1):114-23. doi: 10.1016/j.neo.2014.11.008.
Our recent study showed that SAG/RBX2 E3 ubiquitin ligase regulates apoptosis and vasculogenesis by promoting degradation of NOXA and NF1, and co-operates with Kras to promote lung tumorigenesis by activating NFκB and mTOR pathways via targeted degradation of tumor suppressive substrates including IκB, DEPTOR, p21 and p27. Here we investigated the role of Sag/Rbx2 E3 ligase in cellular senescence and immortalization of mouse embryonic fibroblasts (MEFs) and report that Sag is required for proper cell proliferation and Kras(G12D)-induced immortalization. Sag inactivation by genetic deletion remarkably suppresses cell proliferation by inducing senescence, which is associated with accumulation of p16, but not p53. Mechanistically, Sag deletion caused accumulation of Jun-B, a substrate of Sag-Fbxw7 E3 ligase and a transcription factor that drives p16 transcription. Importantly, senescence triggered by Sag deletion can be largely rescued by simultaneous deletion of Cdkn2a, the p16 encoding gene, indicating its causal role. Furthermore, Kras(G12D)-induced immortalization can also be abrogated by Sag deletion via senescence induction, which is again rescued by simultaneous deletion of Cdkn2a. Finally, we found that Sag deletion inactivates Kras(G12D) activity and block the MAPK signaling pathway, together with accumulated p16, to induce senescence. Taken together, our results demonstrated that Sag is a Kras(G12D)-cooperating oncogene required for Kras(G12D)-induced immortalization and transformation, and targeting SAG-SCF E3 ligase may, therefore, have therapeutic value for senescence-based cancer treatment.
我们最近的研究表明，SAG/RBX2 E3 泛素连接酶通过促进 NOXA 和 NF1 的降解来调节细胞凋亡和血管生成，并通过靶向降解肿瘤抑制性底物（包括 IκB、DEPTOR、p21 和 p27）来与 Kras 合作，通过激活 NFκB 和 mTOR 通路促进肺肿瘤发生。在这里，我们研究了 Sag/Rbx2 E3 连接酶在小鼠胚胎成纤维细胞（MEFs）中的细胞衰老和永生化中的作用，并报告 Sag 是细胞正常增殖和 Kras（G12D）诱导永生化所必需的。通过遗传缺失失活 Sag 可通过诱导衰老显著抑制细胞增殖，这与 p16 的积累有关，但与 p53 无关。从机制上讲，Sag 缺失导致 Sag-Fbxw7 E3 连接酶的底物 Jun-B 积累，Jun-B 是一种转录因子，可驱动 p16 转录。重要的是，Sag 缺失引起的衰老可通过同时缺失 p16 编码基因 Cdkn2a 得到很大程度的挽救，表明其因果作用。此外，通过衰老诱导，Sag 缺失也可以消除 Kras（G12D）诱导的永生化，而这种消除可通过同时缺失 Cdkn2a 得到挽救。最后，我们发现 Sag 缺失失活 Kras（G12D）活性并阻断 MAPK 信号通路，同时积累 p16，从而诱导衰老。总之，我们的研究结果表明，Sag 是 Kras（G12D）合作癌基因，是 Kras（G12D）诱导永生化和转化所必需的，因此靶向 SAG-SCF E3 连接酶可能对基于衰老的癌症治疗具有治疗价值。
